Thailand's revered King Bhumibol Adulyadej is the world's richest royal, according to a Forbes magazine list dominated by Middle Eastern monarchs riding high on surging oil prices.

Thailand's revered King Bhumibol Adulyadej is the world's richest royal, according to a Forbes magazine list dominated by Middle Eastern monarchs riding high on surging oil prices.

Forbes said the 15 monarchs on its list had a combined wealth of $131 billion, up from $95 billion last year, adding that they had managed to hold on to their riches "despite controversy ranging from tax evasion to the dissolution of parliaments in Swaziland and Kuwait".

It did, however, warn that its wealth estimates were "a blend or art and science" because the relationship between individual and state wealth was not always clear.

"While we have tracked the fortunes of a few high-profile royals, like the Queen of England and Sultan of Brunei, for years, this is only the second time we have published a definitive list of the richest royals," Forbes said.

"Monarchs of such countries as Spain and Japan failed to make the cut."

King Bhumibol, 80 years old and the world's longest-reigning monarch after six decades on the throne, is regarded as semi-divine by many Thais.

Forbes said he was worth an estimated $35 billion.

Bhumibol took the top spot from the only other Asian monarch on the list, Sultan Hassanal Bolkiah of oil-rich Brunei, who fell to the number four slot this year after his fortunes shrank from a year ago to $20 billion.

The richest Middle Eastern royal, and the second on the Forbes list, was Sheikh Khalifa bin Zayed al-Nahayan, the president of the oil producing Gulf Arab United Arab Emirates, whose fortune was estimated to be worth $23 billion.

Oil prices which have hit record highs have also helped King Abdullah of Saudi Arabia, the world's top oil exporter, and number three on the list.

Among European monarchs, Prince Hans Adam II of Liechtenstein is the wealthiest, followed by Prince Albert of Monaco and Queen Elizabeth of Britain.

Swaziland's King Mswati III was the only African on the list. Prince Karim Al Husseini, or the Agha Khan, who is the spiritual leader of the Ismaili Muslim sect, is the only royal on the list without an actual country.